1. Field of the Invention
The present invention relates to a high-strength steel sheet for use as an electrode-supporting frame member installed in an aperture grill-type color picture tube and also relates to a manufacturing method thereof.
2. Prior Art
An aperture grill type color picture tube has color selecting electrode elements each made of a thin metal sheet in which striped slits for passage of electron beams are formed at positions corresponding to matrix of RGB (red, green and blue) phosphors arranged on a phosphor screen in order to excite the RGB phosphors with electron beams discharged from an electron gun. The color selecting electrode elements are stretched to and supported with an electrode-supporting frame member.
RGB phosphors on the phosphor screen are selectively irradiated and excited by scanning with electron beams, so as to display a color image composed of red, green and blue colors. Directions of electron beams are likely affected by terrestrial magnetism or the like at the surroundings. If electron beams are deflected by the magnetisms, the color image can not be accurately reproduced. Influence of the terrestrial magnetism or the like can be inhibited by covering the color picture tube with a magnetic shield.
The electrode-supporting frame member installed in the color picture tube is also affected by the terrestrial magnetism or the like, so that its material is important to improve performance of the color picture tube.
The electrode-supporting frame member shall assure location of the slits formed in the color selecting electrode elements for passage of electron beams at accurate positions corresponding to the matrix of phosphors, in order to precisely direct electron beams to each of the RGB phosphors. If the color selecting electrode elements stretched to the electrode-supporting frame member are deviated from predetermined positions due to thermal expansion or the like, the electron beams are not directed to selected phosphors. The mal-direction of the electron beams impedes accurate reproduction of a color image.
The electrode-supporting frame member for supporting the color selecting electrode elements is made from a steel sheet of 2-7 mm in thickness formed into a predetermined shape necessary in response to a size of the color picture tube. An electrode-supporting frame member for a small-sized color picture tube is manufactured by press forming, while an electrode-supporting frame member for a big-sized color, picture tube is commonly manufactured by roll-forming or press-forming a steel sheet at its edges corresponding to four sides of the frame and then fabricating the formed steel sheet to a frame shape fly welding. The electrode-supporting frame member is thereafter stress-relief annealed.
As for the color selecting element, a cold-rolled steel sheet of 0.08-0.15 mm in thickness is etched to form striped slits for passage of electron beams. The etched steel sheet is welded to upper and lower parts of the electrode-supporting frame member which is pressed inwards. After the electrode elements are fixed to the frame member, pressure is removed from the frame member. As a result, the frame member exhibits a reaction force for stretching the electrode elements, due to removal of the pressure. In this way, the electrode elements are fabricated with the frame member.
The frame member integrated with the electrode elements is then subjected to blackening treatment, in order to suppress such defects as heat radiation, generation of secondary electrons and rusts. An oxide film is formed on the frame member by the blackening treatment. The oxide film shall be firmly adhered onto the frame; otherwise oxide particles originated in peeling-off of the black oxide film would scatter in the color picture tube and significantly deteriorate performance of the color picture tube.
In the aforementioned manufacturing process, after the electrode elements are integrated with the frame member, the frame member is subjected to a heat cycle of the blackening treatment under the condition that a bending stress for stretching the electrode elements is applied to the frame member. A tensile force for stretching the electrode elements is reduced due to stress relaxation of both the frame member and the electrode elements which were heated at a high temperature during the blackening treatment.
The structure formed when the electrode elements are stretched to the frame member by application of the tensile force aims at maintaining accurate positional relationship of the electrode elements by the applied tensile force. The tensile force absorbs elongation of the electrode members and eliminates color dislocation, even when the electrode elements are heated and thermally expanded due to radiation with electron beams. However, reduction of the stretching tensile force caused by the blackening treatment weakens function of the frame member and often causes occurrence of color dislocation. Especially, the aperture grill-type color picture tube is likely affected by vibrations due to the striped slits for passage of electron beams which are formed in the electrode elements. When the stretching tensile force is reduced to a certain degree, the electrode elements resonate with acoustic waves from a speaker and cause occurrence of color dislocation.
The frame member is thermally expanded when the electrode elements are heated at a higher temperature. The thermal expansion also causes occurrence of color dislocation.
In this regard, a high-Cr ferritic stainless steel has been used as material for an electrode-supporting frame member, since it is resistant to reduction of a stretching tensile force necessary for the frame member and has a small thermal expansion coefficient. But, use of the ferritic stainless steel causes an increase in a cost of the frame member for a color picture tube.
The inventors proposed an alloyed steel which is cheaper than the ferritic stainless steel, as disclosed in Japanese Patent Application Laid-Open 8-67945. The proposed steel contains 0.01-1.0 wt. % Mo and not more than 2.0 wt. % Cu to improve tensile strength both at a room temperature and a high temperature. Since the steel contains only 3.0 wt. % or less Cr, it is used as a cheaper material instead of a ferritic stainless steel for an electrode-supporting frame member.
By the way, there is a tendency to produce a big-sized color picture tube in these days. Color selecting electrode elements as well as an electrode-supporting frame member become large in size to cope with the size of the color picture tube. Since affections of a stretching tensile force and thermal expansion are greater with enlargement of a frame member in size, it has been inevitable to design a big and heavy electrode-supporting frame member in order to endure the reduction of the stretching tensile force or the thermal expansion.
Taking into consideration the aforementioned properties necessary for an electrode-supporting frame member, there is a strong demand for provision of a new material for the frame member which is improved in strength enough to enable fabrication of an enlightened frame member, good of formability and cheap, and hardly reduces a stretching tensile force.
The present invention aims at provision of such a material, which is cheap, exhibits high strength and hardly reduces a tensile force for stretching color selecting electrode elements even after being subjected to a heat cycle of blackening treatment, and is suitable for use as an electrode-supporting frame member. The object of the present invention is to provide an inexpensive Cr steel sheet which is excellent in strength and formability and has properties necessary for an electrode-supporting frame member instead of an expensive ferritic stainless steel.
A high-strength steel sheet for use as an electrode-supporting frame member according to the present invention consists of 0.003-0.10 wt. % C, not more than 1.0 wt. % Si, 0.05-1.5 wt. % Mn, not more than 0.10 wt. % P, not more than 0.02 wt. % S, 1.5-8.0 wt. % Cr, 0.003-0.10 wt. % Al, and one or more selected from 0.08-0.40 wt. % Ti, 0.08-0.40 wt. % Nb and 0.08-0.40 wt. % V, and the balance being essentially Fe except inevitable impurities. The steel sheet may further contain one or more of Cu up to 2.0 wt. %, Ni up to 2.0 wt. %, 0.01-2.0 wt. % Mo, 0.01-2.0 wt. % W and 0.0003-0.0050 wt. % B.
The steel sheet is manufactured as follows: A slab having the specified composition is hot-rolled at a finishing temperature of 820-950xc2x0 C. and a coiling temperature of 400-700xc2x0 C. A hot-rolled steel strip is skin-pass rolled and then pickled, or the hot-rolled steel strip is pickled and then skin-pass rolled. The steel sheet may be also manufactured by: hot-rolling a slab having the specified composition at a finishing temperature of 820-950xc2x0 C., and coiling temperature of 400-700xc2x0 C., pickling the hot-rolled steel strip, cold-rolling the pickled steel strip, finally annealing the cold-rolled steel strip and then skin-pass rolling the annealed steel strip. A steel sheet for use as an electrode-supporting frame member shall be good of magnetic property, adhesiveness of a black film to a steel substrate and keep a high tensile force necessary for stretching color selecting electrode elements, due to such a structure, quite sensitive to affections derived from reduction of the stretching tensile force as noted in resonance with acoustic waves from a speaker, that the frame member is integrated with color selecting electrode members having striped slits for passage of electron beams formed therein.
The frame member is exposed to a high-temperature atmosphere during blackening treatment under the condition that a bending stress for stretching electrode elements is applied thereto. If a steel sheet lacks in strength at the high temperature, stress relaxation occurs in the frame member. In this point of view, the inventors have researched steel sheets belonging to low-cost common steels but which are excellent in magnetic property, having good adhesiveness of a black film to a steel substrate, and strong at both a room temperature and a high temperature necessary for the frame member.
As a result of our research on various properties of common steels such as thermal expansion, strengthening and high-temperature strength, the inventors have found that addition of Cr together with one or more of Ti, Nb and V and/or one or more of Cu, Ni, Mo, W and B to a low-C steel remarkably strengthens the steel and also increases its high-temperature strength when the steel is subjected to blackening treatment. The increase of strength at both a room temperature and a high temperature suppresses reduction of a tensile force for stretching color selecting electrode elements, so as to provide a steel sheet suitable for a frame member which diminishes color dislocation caused by thermal expansion thereof.
When the new steel sheet is used as material for a frame member for stretching electrode elements, the frame member exhibits sufficient strength at a high temperature so as to suppress reduction of a tensile force for stretching the electrode elements in a state subjected to a heat cycle in a manufacturing process. The steel sheet also has a magnetic property, formability and a blackening, property suitable for the frame member.
Strength at a room temperature, formability, weldability in addition to the high-temperature strength suitable for the frame member of the aperture grill-type color picture tube are bestowed to the steel sheet by manufacturing the steel sheet under conditions specified by the newly proposed method.